Measuring systems and scanning devices of this type are generally known as scanners. They are used in the graphics industry as a means of quality control and for controlling printing processes, for example.
A first known type of scanner has an individual measuring head, which can be moved in one or in two dimensions relative to the measurement object—as a rule a printed sheet. In the situation where the measuring head can be moved in one dimension, the measurement object can be moved in the other dimension. The measuring head respectively scans a small region of the measurement object, what is referred to as an image element or pixel, photoelectrically and each image element is individually scanned by individually moving the measuring head or the measurement object accordingly. The scanning operation may be performed on a densitometric, colorometric or spectral basis, whereby appropriate measurement signals are generated which are then available for processing and/or evaluation. A major disadvantage of these known scanners is the large amount of time involved in scanning a printed sheet of standard size completely, due to the fact of having to move individually the image elements to be measured, which makes them unsuitable for use as a means of automatically controlling or regulating modern printing machines as a rule.
A second known type of conventional scanner is described in patent specification U.S. Pat. No. 6,028,682 (≈DE-A 196 50 223) for example. Scanners of this generic type are equipped with a measurement carriage, which extends transversely across a measuring table in one dimension and can be moved across the second dimension driven by a motor. Disposed in the measuring carriage is a longitudinally extending measuring beam, which contains a large number of measuring heads disposed in a straight line. As the measuring carriage is moved across the measuring table, each measuring head scans the measurement object along a separate scanning track. The measuring heads are provided in the form of pure illuminating and pick-up units and each is connected via an optical multiplexer to a light source and a spectrometer in a time sequence. Although these known scanners are significantly faster than scanners with an individual measuring head mentioned above and are also suitable for applications requiring colorimetric control of a printing process, they are still nevertheless relatively slow on the one hand and mechanically or optically extremely complex on the other hand.
On the basis of the prior art known from patent specification U.S. Pat. No. 6,028,682 (≈DE-A 196 50 223), the objective of this invention is to improve a measuring device and a scanning device of the generic type in terms of scanning speed and design complexity, whilst simultaneously preserving its suitability for quality control purposes in the graphics industry and for colorimetrically controlling printing processes.
As a means of detecting images, i.e. for digitizing documents and similar physical forms, scanners equipped with line scanners are known, which are capable of photoelectrically measuring the respective image elements of a whole image line in one pass. Depending on the design, either the line scanner is moved across the stationary form or the form is moved transversely to the stationary line scanner in order to digitize the entire form line by line. The line scanner comprises a linear light source which is able to illuminate an entire image line simultaneously and a linear sensor array comprising a large number of individual sensors to which the light reflected from the measurement object is directed by what is likewise a linear-shaped optical array. The maximum spatial resolution is theoretically determined by the size of the individual sensors but in practice is reduced to a greater or lesser degree by the effect of scattered light and cross-talk. Color separation takes place either on the basis of a time sequence using either several differently colored light sources (mostly red, blue, green) to illuminate the image lines or a light source which can be switched to different colors, or alternatively an essentially white illuminating light is used with several rows of sensors disposed in parallel, each of which receives light of different wavelength ranges (red, blue, green), which can be set up using upstream filters, for example. Light-emitting diodes (LED) or fluorescent lamps are often used as light sources. Two different systems are commonly used for the arrays of sensors. In the case of the first system, the so-called Contact Image Sensor (CIS), the image lines are imaged by means of gradient index lenses (known as Selfoc Arrays) aligned in rows on a 1:1 scale on the optoelectronic detector rows, and the detector rows comprise optoelectronic line detectors seamlessly aligned in rows (e.g. photodiode arrays). This being the case, the length of the detector rows is identical to the length of the image lines to be scanned. In the case of the second system, the image lines are imaged onto the detector rows by means of a lens and a reducing optical system is often chosen for this purpose, for example imaging on a scale of 1:4. This specifically makes it possible for the length of the detector rows to be smaller than the length of the image lines. If the reduction is sufficient, this makes it possible to set the detector rows up using a single optoelectronic line detector (with a large number of individual sensors) rather than a group of them.
The known scanners of this latter type are very fast and are generally totally satisfactory in terms of image detection. However, if the image elements of the measurement object have to be colorimetrically measured to a high degree of precision, as is generally vital in applications involving quality control and for controlling printing processes, such scanners are not suitable. This is due on the one hand to the cross-talk effects which occur between the individual image elements with scanners of this type and on the other hand to the fact that these scanners are not designed for genuine color measurement processes. When it comes to obtaining correct color and density measurement values, an important aspect is conforming to the measurement geometry prescribed in the relevant standard (typically 45°/0°, e.g. DIN 165361, Part 2), which determines the angle of illumination and reception and permits only a small aperture angle (typically <=5°, e.g. DIN 165361, Part 2). Neither of the commercially available image scanners of these types (CIS and optical imaging system) satisfies these geometric measuring conditions.